In machine tool cutting it is generally desirable that the tool have high spindle rotary speed and power capabilities as in many applications use of the increased available power and higher cutting speeds can provide improved cutting results in terms of the finish and accuracy of the cut workpiece surfaces and the rate of stock removal and machine cycling times. For example, heavy roughing cuts can be performed to maximize the rate of metal removal such as in milling or boring operations, and the resulting rough finish and dimensional variation can be corrected by light finishing cuts. Each of these operations will require that such things as the tool speed, feed rates and depth of cut be carefully selected for minimizing cycle times to improve productivity keeping in mind the rigidity or stiffness of the spindle of the machine and the spindle power limitations on available horsepower.
Typically, the motors for these spindles are induction-type motors which drive a shaft of the spindle carrying a cutting tool with the shaft being supported for rotary motion by bearings. The spindle shaft can be driven by the motor drive shaft with power being transmitted by way of the belts and/or gears of the motor with the tool thus being coupled through the spindle shaft and the motor power transmission gear train to the drive shaft of the motor. Power requirements for machining are proportional to speed and cutting force, and the power lost in bearings and gears of the machine increases with speed. Spindle motor horsepower rating must be considered when selecting a tool because power consumption is in direct relation to the rate of metal removal, which in turn is related to production rates. To minimize the number of cuts required, the depth of cuts should be as great as is possible within the limits of the power of the machine tool and the amount of stock to be removed. As depth of cut increases, the cutting force at the tool head becomes larger. Thus, where relatively deep cuts are desired, a large amount of horsepower may be required. In addition, particular types of machining operations and workpiece materials may dictate certain power requirements such as in tapping of steel where the spindle has to run fairly slowly yet still be capable of providing sufficient torque for cutting. Spindles in many current machine tools have induction motors with limited power capabilities so that cutting forces must be limited to a value that will not overload the machine.
Generally in any machining operation, the power requirement is directly proportional to the material removal rate. The spindle speed is governed by the optimum cutting velocity for that particular workpiece material and tool material combination. The "chip load" is another important factor which influences the tool life. It is established by the spindle speed (rpm) and the feed rate. Hence, in order to maintain an optimum chip load and an optimum cutting velocity for a given power availability at the tool, the depth of cut is the only variable that can be changed. This directly influences the machine time.
For conventional milling operations, depth of cut can be, for example, on the order of 0.250 inches for roughing and 0.025 inches or less for finishing. In milling of aluminum alloys such as for aircraft components, faster and deeper cuts are commonly desired to improve productivity. Depth of cuts several times greater than normal such as on the order of half an inch or greater require greater power capabilities than are provided in many conventional machine tools. As to cutting speeds for aluminum alloys, this is determined by the limits of the machine tool and also by the workpiece. Generally, aluminum can be readily cut at a wide range of speeds. However, as the coefficient of thermal expansion of aluminum alloys can be higher than that of most metals commonly machined, the dimensional accuracy of finished parts requires that the part be kept cool during machining. High cutting speed helps keep the part cool, because most of the heat introduced into the part during a given rotation is removed with the chip during the next rotation, and the time for diffusion of the heat into the part is short. In addition, high speeds will generally yield a high rate of metal removal and produce the best finish. Thus, if the aluminum part to be cut can take high speed cutting, this is generally desirable. However, mainly because of limitations imposed by available spindle speed and horsepower, very high speed cutting operations are not commonly done on aluminum parts. Thus, a machine tool having a spindle capable of producing deep cuts and fast speeds is desirable, particularly for minimizing temperature rise in aluminum parts during machining thereof.
Spindle stiffness has a marked effect on the maximum speed that will not cause chatter. Chatter is a condition in which the machine tool cutter vibrates in resonance at a frequency determined by the natural frequency of the spindle shaft. In other words, spindles have their own characteristic or critical speed at which vibrations get very high. Spindles cannot be operated at their critical speed as otherwise harmonic vibration can occur. Chatter as caused by resonant or harmonic vibrations adversely affects machining accuracy in terms of tolerances and part finish. In addition, the stiffness of a spindle shaft is a significant factor in the amount of deflection caused by cutting forces. Tool deflection and chatter resulting from lack of spindle rigidity can cause excessive tool wear and breakage, damage to workpieces, dimensional inaccuracy and unacceptable surface finish.
Current spindles used for high speed cutting operations can be relatively large and have unfavorable length to diameter ratios, and the spindle design and the bearings therefor generally cannot provide for the desired stiffness especially as the bearing surfaces start to wear or fatigue creating pitting in these surfaces due to the high frequency vibrations and extreme load bearing conditions which can be generated at high speeds. To avoid the onset of resonant vibrations in these spindle units, lower spindle speeds are typically employed. However, in some instances for example, it may be desirable to cut at high speeds with lighter feeds to improve the part finish. Stiff spindles are necessary when low-micro-inch finishes must be achieved. Thus, it is desirable to design machine tools with stiff spindles to avoid the onset of resonant vibrations causing chatter to minimize or eliminate the problems resulting from high frequency vibrations and chatter thereby increasing production and lowering production costs. The increased speed possible with a stiff spindle design can improve finishes and increase production rates.
For machine tools, due to the considerable number of parts constituting the bearings, it is extremely difficult to provide high speed spindles with rolling-contact bearings affording a high degree of machining precision and yielding very satisfactory surface conditions. The use of conventional bearings such as roller or ball bearings for supporting the loads taken by the spindle shaft, particularly in high speed applications, can cause problems with accuracy as they fatigue under the high centrifugal forces and the extreme friction, heat and load bearing conditions which can be generated at high speeds. In addition, the life of these contacting bearings under a given load is typically a certain number of revolutions so that high speeds cause this number of revolutions to be used up at a relatively high rate correspondingly shortening the life of rolling element bearings. A combination of hydrodynamic and hydrostatic bearings to support a high speed spindle is disclosed in U.S. Pat. No. 5,462,364, wherein the hydrodynamic bearings are effective at high speed rotation, and the hydrostatic bearings are effective at low speed rotations of the spindle. Fluid hydrodynamic and hydrostatic bearings can generally deliver substantially greater performance than conventional rolling element bearings; specifically, these fluid bearings are insensitive to surface imperfections in the bearing, they normally will not wear over time, they have a large load capacity, and they are substantially immune to momentary overloads which could cause rolling element bearings to indent and cause pitting in bearing surfaces.
In hydrodynamic bearing spindles, the stiffness varies as the speed varies; while in typical hydrostatic bearings, hydraulic oil under pressure provides a more uniform stiffness. Thus, the position of the spindle in relation to the fixed bearing member is independent of the rotary speed of the spindle shaft supported by hydrostatic bearings as long as the maximum permissible bearing load value as determined by the pumping pressure and bearing configuration is not attained so that no contact will take place between the spindle shaft and fixed bearing. Despite these advantages, the use of oil under pressure in the bearings presents problems in terms of sealing of the bearing fluid in the spindle so that it does not mix with cutting fluids, particularly at high speeds where use of seals in contact with rapidly rotating parts can tend to wear thus losing their sealing ability. Many cutting fluids are water-oil emulsions and any contamination of the cutting fluid with incompatible fluids such as hydraulic oils for hydrostatic bearings can cause problems ranging from creating excessive variation in workpiece finish or dimensions to shortening of tool life. Sealing problems can be exacerbated where the spindle is used in a nutator type machine tool with the spindle changing attitudes with respect to the horizontal so as to increase the effects of gravitational forces on the bearing fluid tending to draw it out from the spindle when the spindle tilts. Accordingly, there is a need for a cutting spindle and motor apparatus that maintains a relatively constant stiffness and load capacity at low cutting speeds and at high cutting speeds, e.g., 20,000 rpms or more. For such a spindle and motor apparatus, a spindle sealing system is needed which prevents significant leakage of oil from fluid bearings in the spindle to the cutting fluid at the machining area at high operating speeds of the machine tool and when the spindle is stationary.
Hydrostatic bearings are commonly designed with circumferentially spaced pockets into which fluid under pressure is fed. The pockets are surrounded by a land or sill area, and typically a drain groove is formed in the sill area between adjacent pockets. One problem that has been found with hydrostatic fluid bearings run at high rotary speeds is the lowering of the fluid pressure in the pockets reducing the load bearing capacity of the hydrostatic bearings. It is believed that this reduction of the bearings load bearing capacity is due to reduction in viscosity of oil because of increased turbulence and fluid friction generated in the bearing pockets at high rotary speeds causing energy losses and lowering of fluid pressure in the pockets. Above certain high rpms of spindle rotation, the so-called "viscous pumping action" of the bearing fluid becomes predominant, and oil in the pocket is pumped out in considerable quantities; and in the absence of replenishing oil, the pocket pressure is reduced, thereby drastically affecting the hydrostatic capacity of the fluid bearings, which is undesirable.
In this regard, it is important to maintain certain minimum clearances in the bearings to keep their load bearing capacity sufficient for the increased loads experienced at the high rotary speeds at which the present spindle can be run. In addition to viscous pumping of bearing fluid, where high temperatures cause thermal expansion of the spindle, the clearances preset into fixed bearings may be decreased past their minimum tolerances necessary for proper load bearing capacity, and machining errors due to the thermal deformation of the spindle components can occur.